Conversion functions

Just like fromList both of these functions convert an association
list into a trie, with earlier values shadowing later ones when
keys conflict. Depending on the order of keys in the list, there
can be as much as 5x speed difference between the two. Yet,
performance is about the same when matching best-case to best-case
and worst-case to worst-case (which is which is swapped when
reversing the list or changing which function is used).

This version sorts the list before folding over it. This adds
O(n log n) overhead and requires the whole list be in memory
at once, but it ensures that the list is in best-case order. The
benefits generally outweigh the costs.